1. University of Innsbruck, Technickerstrasse 25/8, Innsbruck, A-6020, Austria
2. SOFIA Science Center, USRA, NASA Ames Research Center, M.S. N232-12, Moffett Field, CA 94035, USA
3. INAF-Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34143 Trieste, Italy
4. UPMC-CNRS, UMR7095, Institut d'Astrophysique de Paris, 98brs Bd Arago, F-75014, Paris, France